1. Field of Invention
The present invention relates to a method for sensing a paper skew, and more particularly to a method for sensing a paper skew by employing existing sensors of a device having a paper feeding mechanism, in which the sensing result further is utilized for correcting the paper skew.
2. Related Art
Nowadays a device (such as a printer, copier or scanner) generally has a paper feeding mechanism for feeding a paper into the device and then printing or scanning. However, any paper feeding mechanism has a problem of paper skew while feeding. As a result, the contents printed on the paper or scanned from the paper are skewed, and even need to print or scan again. It may waste resources. Therefore, various mechanisms for sensing and correcting the paper skew have been developed.
Referring to FIG. 1, there is shown a sketch diagram illustrating the position relationship of a paper and sensors in accordance with a conventional method for sensing and correcting the paper skew disclosed by U.S. Pat. No. 6,895,210. In FIG. 1, the direction of y is a direction of feeding a paper 10 (which is called the direction of paper feeding thereinafter), and the direction of x is perpendicular to the direction of y. In the conventional method, both sensors 11 and 12 are configured on a line parallel to the direction of x, and both the sensor 12 and a sensor 13 are configured on another line parallel to the direction of y. The sensor 11 and 12 are used to measure skew, and the sensors 12 and 13 are used to measure velocity. After the paper 10 is fed, because of the paper skew, the paper 10 will be sensed by the sensor 11 at time t1, and then sensed by the sensor 12 at time t2, and finally sensed by the sensor 13 at time t3. Therefore, a skew angle θ of the paper 10 is tan−1 [d2/d1*(t2−t1)/(t3−t2)] in radian, where d1 is the distance between the sensors 11 and 12, and d2 is the distance between the sensors 12 and 13.
The measured skew angle θ of the paper 10 can be employed by the printer, copier or scanner to modulate the angle of the paper 10 for compensating the paper skew in the hardware manner, or to modulate the contents to be printed on the paper 10 or scanned from the paper 10 for compensating the paper skew in the software manner. However, the method for sensing and correcting the paper skew disclosed by the patent additionally needs three sensors for sensing the paper 10 and a timer for measuring the time t1-t3.